1. Field of the Invention
The present invention relates to a measurement apparatus, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
An exposure apparatus has conventionally been employed to fabricate micropatterned semiconductor devices such as a semiconductor memory and logic circuit by using photolithography. The exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
A minimum line width (resolution) that the exposure apparatus can transfer is proportional to the exposure light wavelength and is inversely proportional to the numerical aperture (NA) of the projection optical system. Along with the recent demand for increasing the packing density and the degree of integration of semiconductor devices, the exposure apparatus is required to attain a higher resolution. To meet this demand, the exposure light wavelength is shortening, and the NA of the projection optical system is increasing. For example, to shorten the exposure light wavelength, a KrF excimer laser (with a wavelength of about 248 nm) or an ArF excimer laser (with a wavelength of about 193 nm) has come to be used as the exposure light source in place of a superhigh pressure mercury lamp. To increase the NA of the projection optical system, an immersion exposure technique is attracting a great deal of attention. This technique increases the NA of the projection optical system by filling the space between the projection optical system and the wafer with a liquid, which has a refractive index higher than 1, such as pure water.
As the resolution of the exposure apparatus increases, a stricter demand has arisen for the measurement, adjustment, and correction of the optical characteristic of the projection optical system. For example, it is demanded to precisely measure even a change in optical characteristic due to heat generated during exposure and that due to a slight change in lens position, which occurs in the process of transporting the exposure apparatus. The optical characteristic of the projection optical system must be measured by the exposure apparatus while the projection optical system is mounted in the exposure apparatus, and be adjusted to an optimal state. Note that the optical characteristic herein does not depend on the polarization of the exposure light, and represents the phase difference from an ideal state, as in the conventional wavefront aberration. This optical characteristic will be referred to as “non-polarization aberration” hereinafter).
The polarization state of the exposure light is becoming an important factor along with an increase in the NA of the projection optical system, so the polarization of the exposure light must be controlled. However, even when the polarization state of the exposure light is controlled to a desired one by the illumination optical system, the polarization state of the exposure light in the projection optical system often changes due to, for example, the holding of a lens, the deformation of a lens due to heat, the use of a birefringent glass material, or the reflection of the exposure light by a mirror. Naturally, the necessity of precisely knowing the polarization state of the exposure light in the projection optical system is growing. The optical characteristic of the projection optical system attributed to its birefringence depends on the polarization of the exposure light, and will be referred to as “polarization aberration” hereinafter. The polarization aberration includes the retardation and azimuth (along the fast axis and slow axis) of birefringence. However, there are various known polarization aberration expression methods such as the Stokes parameters, Jones matrix, and Mueller matrix, in addition to the retardation and azimuth.
In this manner, the recent exposure apparatus must measure the non-polarization aberration, which does not depend on the polarization of the exposure light, and the polarization aberration, which depends on the polarization of the exposure light, of the projection optical system as its optical characteristics with high accuracy in a short period of time. Under the circumstance, several techniques of measuring the non-polarization aberration and polarization aberration of an optical system have been proposed.
For example, Japanese Patent Laid-Open Nos. 2-116732, 2002-71515, and 2004-257854 disclose techniques using an interferometer dedicated to measurement, which utilizes interference between the reference wavefront and the wavefront to be measured. PCT(WO) 2003/028073 discloses a technique using an aberration measurement unit which measures the non-polarization aberration, and a polarization conversion unit which roughly calculates the polarization aberration. Japanese Patent Laid-Open No. 2005-116732 discloses a technique of calculating the Mueller matrix as the polarization aberration of the projection optical system. Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 disclose techniques of measuring the polarization aberration of the projection optical system using an interferometer.
Unfortunately, these prior arts cannot simultaneously measure the non-polarization aberration and the polarization aberration (the retardation and azimuth of birefringence), while the projection optical system is mounted in the exposure apparatus, with high accuracy in a short period of time. Particularly in recent years, since the exposure light readily heats up an optical system so that the non-polarization aberration and polarization aberration change with time, the non-polarization aberration and polarization aberration must be measured in a shorter period of time in response to these changes. Also, the required accuracy of measuring the non-polarization aberration and polarization aberration is becoming stricter along with advance of the micropatterning of semiconductor devices.
For example, since the interferometers dedicated to measurement, which are disclosed in Japanese Patent Laid-Open Nos. 2-116732, 2002-71515, and 2004-257854, each use a relatively long, complicated optical path that does not serve as a common path, the influences of vibration on them are non-negligible. Still worse, it is difficult to apply such an interferometer to measurement on the exposure apparatus because the exposure light has a very short coherence length.
PCT(WO) 2003/028073 proposes a technique of measuring the optical characteristics of an optical system by two orthogonal linearly polarized light beams using an aberration measurement unit and polarization conversion unit. However, the use of only the two orthogonal linearly polarized light beams is insufficient to precisely measure the polarization aberration of the projection optical system. Note also that the aberration measurement unit and polarization conversion unit each include a CCD as an image sensing element, and the CCD is inserted/removed into/from the exposure apparatus. This may make it impossible to measure the optical characteristics with high accuracy as the positional relationship between the optical axis and the CCD in the exposure apparatus varies. Also, since the aberration measurement unit and polarization conversion unit must be aligned before the measurement, a correspondence between the non-polarization aberration and the polarization aberration may shift depending on the alignment accuracy. These two units include optical system. When attaching and detaching these whole units including the optical system, very high alignment accuracy with the optical axis of the optical system is required. Since PCT(WO) 2003/028073 requires two units, that is, the aberration measurement unit and polarization conversion unit, the cost increases.
Japanese Patent Laid-Open No. 2005-116732 discloses an application of a general technique of inserting a phase shifter, polarizer, and light-receiving element in turn at the succeeding stage of an optical system to be measured, and rotating the phase shifter to detect a change in intensity, thereby detecting the polarization state of the optical system to be measured. Japanese Patent Laid-Open No. 2005-116732 can precisely calculate the Mueller matrix as the polarization aberration, but does not disclose a technique of measuring the non-polarization aberration.
Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 disclose techniques of measuring the polarization aberration of the projection optical system using an interferometer, as described above. These techniques exploit, in particular, shearing interference. Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 disclose techniques of measuring the polarization aberration, but do not disclose the measurement conditions and techniques of measuring the non-polarization aberration. When a polarizer is inserted at the preceding stage of a photodetector, the light amount greatly changes depending on the relationship between the polarization state of the incident light beam and the rotation angle of the polarizer. For example, assuming that the incident light beam is linearly polarized, the light amount becomes a maximum if the polarization direction matches the transmission axis of the polarizer, while it becomes a minimum if the polarization direction is perpendicular to the transmission axis of the polarizer. The light amount becomes particularly small when the birefringence amount is small. Because the measurement accuracy and the light amount have a close relationship, the optical characteristics cannot be measured unless an optimal set of measurement conditions can be ensured.
Neither of the techniques disclosed in Japanese Patent Laid-Open Nos. 2004-61515 and 2006-237617 provides a system in which the exposure apparatus can measure the reference polarization aberration which serves as a reference in measuring the polarization aberration. These techniques cannot cope with a temporal change in reference polarization aberration and therefore cannot measure the polarization aberration of the projection optical system with high accuracy. Even when the reference polarization aberration is measured in advance by another measurement system, it is different from that on the exposure apparatus due to the influence of, for example, a thermal change of a lens by the exposure light or the deformation of a lens in the process of mounting the projection optical system into the exposure apparatus.